Method for producing lithium tantalate crystal

ABSTRACT

A method of producing a lithium-tantalate crystal, wherein at least a first material containing lithium tantalate, lithium niobate or hydrogen storage alloy storing hydrogen that is subjected to a heat treatment at a temperature of T1′ that is Curie temperature or higher in a reducing atmosphere is superposed on a single-polarized lithium-tantalate crystal, and then the crystal is subjected to a heat treatment at a temperature of T2′ that is lower than Curie temperature in a reducing atmosphere, thereby an electric conductivity of the single-polarized lithium-tantalate crystal is increased. There can be provided a method of producing a lithium-tantalate crystal wherein the surface charge generated by applying a temperature change to the lithium-tantalate crystal can be decayed quickly without accumulating by increasing the electric conductivity, and an effective piezoelectric property is exhibited by maintaining the single polarized structure.

TECHNICAL FIELD

The present invention relates to a method of producing alithium-tantalate crystal which is used for the application wherein apattern is formed with a metal electrode on a wafer to process anelectrical signal such as a surface acoustic wave device.

DESCRIPTION OF THE RELATED ART

Lithium tantalate is used for the application which utilizes itselectrical properties, such as a SAW device which performs a signalprocessing using a surface acoustic wave (SAW). The lithium-tantalatecrystal suitable for this purpose shows a piezoelectric response(piezoelectric property) originated from the crystal structure thereofand needed for a SAW device. However, the lithium-tantalate crystalobtained by a general method has a pyroelectric response (pyroelectricproperty), in addition to the piezoelectric property.

A piezoelectric property of a lithium-tantalate crystal is essentialcharacteristic when using the lithium-tantalate crystal as a SAW device.On the other hand, a pyroelectric property is observed as a surfacecharge generated on an external surface of a lithium-tantalate crystalby applying a temperature change to the crystal, and charges the crystalwith electricity. It is considered that, when using a lithium-tantalatecrystal as a SAW device, the surface charge causes a spark dischargebetween metal electrodes formed on the wafer consisting of alithium-tantalate crystal, which causes a significant defect in aperformance of the SAW device. For this reason, in a design of a SAWdevice using a lithium-tantalate crystal, there are needed an artificefor preventing the generation of a surface charge, an artifice fordischarging the surface charge, an artifice for making an intervalbetween metal electrodes large, or the like. There is the disadvantagethat the design of the SAW device itself is restricted in order to takethese artifices in the design.

Moreover, in the process of producing a SAW device using alithium-tantalate crystal, there is a process of heating thelithium-tantalate crystal in processes such as deposition of a metalfilm and removal of a resist. If a temperature change such as elevationor lowering of temperature is applied to the lithium-tantalate crystalin these processes, a charge will be generated on the external surfacedue to the pyroelectric property of the lithium-tantalate crystal. Asmentioned above, a spark discharge may be generated between metalelectrodes due to this surface charge, and cause the breakage of theelectrode pattern. Therefore, artifices are considered so that atemperature change may not be applied as possible, or so that atemperature change may be gentle in the process of producing the SAWdevice. These artifices may cause disadvantages that the throughput ofthe production process is lowered, and that the margin which guaranteesthe performance of the SAW device becomes narrow.

Although the charge on the external surface generated due to thepyroelectric property is neutralized by the free charge from thesurrounding environment and decayed with time in the lithium-tantalatecrystal produced by the usual method, the decay time is long as severalhours or more, and thus it is not industrial to decay the generatedsurface charge by such natural neutralization in the process ofproducing the SAW device.

From the above-mentioned background, there has been increased a demandfor the piezoelectric crystal in which the generation or accumulation ofthe charge is not observed on the external surface of the crystal withmaintaining the piezoelectric property needed, in order to achievedevice characteristics for the application like a SAW device, and therehas been needed a lithium-tantalate crystal in which the accumulation ofthe surface charge is not observed for such an application.

In order to prevent the accumulation of the surface charge, it wasconsidered to increase the electric conductivity of a lithium-tantalatecrystal. As a method for producing a lithium-tantalate crystal in whichthe electric conductivity is increased, for example, in Japanese PatentApplication Laid-open (Kokai) No.11-92147, there is disclosed a methodwherein a lithium-tantalate crystal is exposed to a reducing atmosphereat 500° C. or higher. However, if the lithium-tantalate crystal isreduced according to the method as described above, the single polarizedstructure needed for the application to the SAW device is lost in thecase that the treatment temperature in a reducing atmosphere is 610° C.,which is the Curie temperature of lithium tantalate, or higher. In thecase that the treatment temperature in a reducing atmosphere is 610° C.or less, a reaction rate of the reduction treatment becomes extremelyslow. As a result, it was found that by using the method the electricconductivity of a lithium-tantalate crystal cannot be increasedindustrially.

DISCLOSURE OF THE INVENTION

The present invention provides a method to solve the above-mentionedproblems and the present invention provides a method of producing alithium-tantalate crystal in which the surface charge generated byapplying a temperature change to the lithium-tantalate crystal can bedecayed without accumulating by increasing the electric conductivity ofthe lithium-tantalate crystal, and an effective piezoelectric propertyis exhibited by maintaining the single polarized structure.

To solve the above-mentioned problems, the present invention provides amethod of producing a lithium-tantalate crystal with increased electricconductivity, wherein a substance reduced at a temperature of T1 iscontacted with a lithium-tantalate crystal at a temperature of T2 thatis lower than the temperature of T1 in a reducing atmosphere. Accordingto the method, by reducing a lithium-tantalate crystal using a reducedsubstance, the electric conductivity of the lithium-tantalate crystalcan be made high at relatively lower temperature. As a result,pyroelectric charge generated by applying a temperature change to thelithium-tantalate crystal can be reduced.

Moreover, it is preferable that the temperature of T1 is 700° C. orhigher. Thereby, the reduction treatment of the substance can be quicklyconducted.

In addition, it is preferable that the reduction at the temperature ofT1 is performed in a reducing gas containing any one of hydrogen, carbonmonoxide and nitrous oxide, or a mixed gas consisting of two or more ofthem. When performing the reduction like this, the reduced substance canbe obtained by using generally known reducing gases. It is especiallypreferable to use hydrogen or carbon monoxide as the reducing gasbecause the reduction treatment can be quickly conducted.

Furthermore, the reduction is also preferably performed in an atmospherein which any one of He, Ne, Ar, other rare gases, nitrogen, and carbondioxide, or a mixed gas consisting of two or more of them is addedfurther to the reducing gas. By performing the reduction like this, arate of the reduction and a treatment time for the reduction can becontrolled.

In addition, the reducing gas atmosphere is preferable that can treat atarget substance for the reduction treatment as quickly as possible.

Furthermore, in the present invention, a crystal, a ceramic, a metal andpreferably a hydrogen storage alloy can be used as the substance reducedat the temperature of T1. As mentioned above, if a substance that canreduce a lithium-tantalate crystal at the temperature of T2 is used asthe substance reduced at the temperature of T1, a lithium tantalate withincreased electric conductivity can be produced.

In addition, in the present invention, the crystal or the ceramic whichconsists of mixed oxide with nonstoichiometric composition can be used.By using this, a substance reduced effectively at the temperature of T1can be obtained. This is because nonstoichiometric composition has lackof cations and it is considered that the lack is closely related to thereduction treatment.

Instead of using the stoichiometric composition having a little lack ofcations, if a composition that doesn't have a composition ratioaccording to stoichiometry such as congruent composition is used, highreducing property can be achieved because of a lot of lack of cations.

Furthermore, a lithium tantalate or a lithium niobate can be used as thecrystal or the ceramic. It is preferable to use these because they areconventionally used as a material for the SAW device, they don'tcontaminate a lithium-tantalate crystal to be a product and they havesufficient reducing property.

In addition, in the present invention, a single-polarized crystal can beused as the lithium-tantalate crystal contacted at the temperature of T2with the substance reduced at the temperature of T1. It is preferable touse this because the lithium-tantalate crystal obtained according to thepresent invention isn't necessary to be subjected to a singlepolarization process after the reduction treatment at the temperature ofT2.

A conventional single polarization process is conducted at a hightemperature of Curie temperature of lithium tantalate (about 610° C.) orhigher and by applying a voltage in an atmosphere. However, when alithium-tantalate crystal in which electric conductivity is increased bya reduction treatment or the like is made to a temperature of 400° C. orhigher in an atmosphere, the increased electric conductivity is lowered.As a result, there is a problem that even if a lithium-tantalate crystalis subjected to a reduction treatment to increase its electricconductivity, the electric conductivity reverts to the value before thereduction treatment by conducting a following single polarizationprocess at Curie temperature or higher.

Therefore, in the present invention, it is preferable that thetemperature of T2 in which a lithium-tantalate crystal and a reducedsubstance are contacted with each other is 400° C. -600° C. As describedabove, if T2 is lower temperature than the Curie temperature of alithium-tantalate crystal, a single-polarized lithium-tantalate crystalcan maintain the single polarized structure, and because the contact isconducted in a reducing atmosphere, there is no problem that theelectric conductivity is lowered.

In addition, it is preferable that after the process at the temperatureof T2, an atmosphere is introduced at a temperature of 250° C. or less.As mentioned above, by being exposed to an atmosphere at a temperatureof 250° C. or less, there is no possibility that electric conductivityof the lithium-tantalate crystal increased by the treatment at thetemperature of T2 is lowered by being exposed to the atmosphere.

Furthermore, a crystal that is in a step before slicing can be used asthe single-polarized crystal, and a wafer subjected to a slicing processor a wafer subjected to a lapping process can be used as thesingle-polarized crystal. Especially, when a sliced wafer or a lappedwafer is used, a surface area compared to a volume becomes larger and soa contact area with the reduced substance can be increased, thus theelectric conductivity can be increased effectively.

In addition, in the present invention, it is preferable that thereduction at the temperature of T2 is performed in a reducing gasconsisting of any one of hydrogen, carbon monoxide and nitrous oxide, ora mixed gas consisting of them. Thereby, a reduction treatment isconducted in a generally known reducing gas and a lithium-tantalatecrystal with increased electric conductivity can be obtained. It isespecially preferable to use hydrogen or carbon monoxide as the reducinggas because the reduction treatment can be quickly conducted.

Furthermore, the reduction at the temperature of T2 is performed in anatmosphere in which an inert gas consisting of He, Ne, Ar, other raregases, nitrogen, and carbon dioxide, or a mixed gas consisting of themis added further to the reducing gas consisting of any one of hydrogen,carbon monoxide and nitrous oxide, or a mixed gas consisting of them. Byperforming the reduction like this, a rate of the reduction and atreatment time for the reduction can be controlled.

The present invention also provides a method of producing alithium-tantalate crystal, wherein at least a first material containinglithium tantalate, lithium niobate or hydrogen storage alloy storinghydrogen that is subjected to a heat treatment at a temperature of T1′that is Curie temperature or higher in a reducing atmosphere issuperposed on a single-polarized lithium-tantalate crystal, and then thecrystal is subjected to a heat treatment at a temperature of T2′ that islower than Curie temperature in a reducing atmosphere, thereby anelectric conductivity of the single-polarized lithium-tantalate crystalis increased.

According to the method, because the first material has reducingproperty, by superposing the first material on the single-polarizedlithium-tantalate crystal and subjecting them to a heat treatment at atemperature of T2′ that is lower than Curie temperature in a reducingatmosphere, a single-polarized lithium-tantalate crystal can be reducedeffectively even if the temperature is not Curie temperature or higher.Accordingly, the electric conductivity of a lithium-tantalate crystalcan be effectively increased with the crystal being single-polarized.

In addition, it is preferable that the lithium-tantalate crystalobtained by the heat treatment at the temperature of T2′ is subjected toa heat treatment at a temperature of T3 that is lower than Curietemperature in an atmosphere, the lithium-tantalate crystal issuperposed on a second material containing lithium tantalate, lithiumniobate or hydrogen storage alloy storing hydrogen that is subjected toa heat treatment at a temperature of T1″ that is Curie temperature orhigher in a reducing atmosphere, and the crystal is subjected to a heattreatment at a temperature of T2″ that is lower than Curie temperaturein a reducing atmosphere, thereby an electric conductivity of thelithium-tantalate crystal is increased.

When a single-polarized lithium-tantalate crystal in which the electricconductivity is increased by the reduction according to theabove-mentioned method is once subjected to a heat treatment at atemperature of T3 that is lower than Curie temperature in an atmosphere,and then the crystal is superposed on a second material having reducingproperty and subjected to a heat treatment at a temperature of T2″ thatis lower than Curie temperature in a reducing atmosphere, it is surethat the electric conductivity of the lithium-tantalate crystal can beincreased more uniformly.

In addition, it is preferable that the lithium-tantalate crystalobtained by the heat treatment at the temperature of T2′ is subjected toa heat treatment at a temperature of T3 that is lower than Curietemperature in an atmosphere, the lithium-tantalate crystal is subjectedto a heat treatment at a temperature of T2′″ that is lower than Curietemperature in a reducing atmosphere, thereby an electric conductivityof the lithium-tantalate crystal is increased.

As mentioned above, when a single-polarized lithium-tantalate crystal inwhich the electric conductivity is increased by the reduction accordingto the above-mentioned method is once subjected to a heat treatment at atemperature of T3 that is lower than Curie temperature in an atmosphere,and then without using the second material the crystal is subjected to aheat treatment at a temperature of T2′″ that is lower than Curietemperature in a reducing atmosphere, it is sure that the electricconductivity of the lithium-tantalate crystal can be increased.

In this case, it is preferable that the temperatures of T2′, T2″, T2′″that are lower than Curie temperature are 400° C. or higher. Thereby,the electric conductivity of a lithium-tantalate crystal can beeffectively increased.

In addition, it is preferable that a wafer subjected to a slicingprocess or a wafer subjected to a lapping process is used as thesingle-polarized lithium-tantalate crystal. When using this, since asurface area compared to a volume becomes larger and a contact area withthe first or the second material or an area that is exposed to areducing atmosphere can be larger, the electric conductivity of alithium-tantalate crystal can be effectively and uniformly increased.

Furthermore, it is preferable that a ceramic or a crystal consisting oflithium tantalate or lithium niobate is used as the first material orthe second material. As described above, either a ceramic or a crystalof lithium tantalate or lithium niobate can be used as the firstmaterial or the second material. Such a material doesn't cause harmfuleffect such as contamination or others when it is superposed on thelithium-tantalate crystal and subjected to a heat treatment.

In addition, it is preferable that a composition that doesn't have acomposition ratio according to stoichiometry is used as the ceramic orthe crystal consisting of lithium tantalate or lithium niobate. Whenusing this, because the first or the second material has much lack ofcations and reducing property becomes high, the electric conductivity ofa lithium-tantalate crystal can be effectively and uniformly increased.

Furthermore, it is preferable that a wafer subjected to a slicingprocess that is obtained from a crystal consisting of lithium tantalateor lithium niobate, or a wafer subjected to a lapping process that isobtained from a crystal consisting of lithium tantalate or lithiumniobate is used as the first material or the second material. When usingthis, because a surface area compared to a volume becomes larger and anarea that is exposed to a reducing atmosphere can be larger, thematerial effectively reduced can be obtained. In addition, because sucha material is easy to be superposed on a single-polarizedlithium-tantalate crystal, the electric conductivity of alithium-tantalate crystal can be effectively increased.

In addition, it is preferable that after the heat treatment at thetemperature of T2′, T2″ and T2′″, the lithium-tantalate crystal isexposed to an atmosphere at a temperature of 250° C. or less. Thereby,there is no possibility that the electric conductivity of asingle-polarized lithium-tantalate crystal increased by the heattreatment at the temperature of T2′, T2″ and T2′″ is lowered by beingexposed to an atmosphere.

Furthermore, it is preferable that the reducing atmosphere for the heattreatment at the temperature of T1′ , T1″, T2′, T2″ and T2′″ contains areducing gas consisting of at least any one of hydrogen, carbon monoxideand nitrous oxide. As mentioned above, any one of these conventionallyknown reducing atmospheres can be used as a reducing atmosphere of theheat treatment conducted at either temperature.

Moreover, it is preferable that an additional gas consisting of at leastany one of rare gases, nitrogen, and carbon dioxide is added further tothe reducing gas. By doing as mentioned above, a rate of the reductionand a treatment time for the reduction can be controlled.

As explained above, according to the present invention, alithium-tantalate crystal in which accumulation of the surface chargegenerated by a temperature change due to pyroelectric property is notsubstantially observed, especially a single-polarized lithium-tantalatecrystal can be produced because electric conductivity of the crystal isincreased. Accordingly, a lithium-tantalate crystal in whichaccumulation of the charge is not observed on the external surface ofthe crystal with maintaining the piezoelectric property thereof, andthus it is very advantageous for the production of the SAW device can beproduced. Furthermore, the method of the present invention isindustrially advantageous because the above lithium-tantalate crystalscan be produced effectively by a quite short-time process.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be further described below in detail.

As described above, there is disclosed a method that a lithium-tantalatecrystal is exposed to a reducing atmosphere at 500° C. or higher as aconventional method of producing a lithium-tantalate crystal withincreased electric conductivity.

However, when a lithium-tantalate crystal is reduced according to such amethod, there is a problem that the single-polarized structure neededfor a SAW device application is lost if treatment temperature in areducing atmosphere is 610° C. which is the Curie temperature of lithiumtantalate or higher, and that the reaction rate of the reductiontreatment becomes very slow if the temperature is 610° C. or less, andtherefore it isn't industrial.

The inventor of the present invention has found that when a firstmaterial with reducing property containing lithium tantalate, lithiumniobate or hydrogen storage alloy storing hydrogen that is reducedeffectively at a temperature that is Curie temperature or higher in areducing atmosphere is superposed on a single-polarizedlithium-tantalate crystal, and then the crystal is subjected to a heattreatment in a reducing atmosphere, the first material reduces thelithium-tantalate crystal via the interface, thereby the reductiontreatment can be effectively conducted even at a temperature lower thanCurie temperature, electric conductivity can be increased and the singlepolarized structure can also be maintained. In this case, it isconsidered that the reduction treatment activates the single-polarizedlithium-tantalate crystal to a reduction reaction.

Moreover, the inventor of the present invention has found that when thelithium-tantalate crystal activated to the reduction reaction issubjected to a heat treatment at a temperature that is lower than Curietemperature in an atmosphere, electric conductivity is once decreased(resistivity is increased) but the activated state is maintained,therefore the crystal superposed on a second material with reducingproperty or the crystal alone is subjected to a heat treatment at atemperature that is lower than Curie temperature in a reducingatmosphere, thereby electric conductivity can be further increased(resistivity can be further decreased). And then the inventor completedthe present invention.

The embodiments of the present invention will be specifically explainedbelow, but the present invention is not limited thereto.

As an example of a substance to be reduced at a temperature of T1, afirst material or a second material that is used in the presentinvention (hereinafter occasionally referred to as materials), there isa ceramic consisting of lithium tantalate. This can be obtained byweighing and mixing lithium carbonate and tantalum pentoxide, andheating them at 1000° C. or higher in an electric furnace. A ceramicconsisting of lithium niobate can be used since it is obtained by usingniobium pentoxide instead of tantalum pentoxide.

The ceramic obtained above is put into a stainless steel container or acontainer made of quartz. The container is placed in a sealed furnace. Areducing gas is flowed at a rate of approximately 1.5 liters per minutethrough the sealed furnace. A temperature of the furnace is increasedfrom room temperature to a temperature of T1, or a temperature of T1′ orT1′ that is Curie temperature or higher like 700° C.-1200° C. Aftermaintaining the temperature for 1-50 hours, the temperature of thefurnace is decreased at a rate of around 6.7° C. per minute and thecontainer is removed from the furnace. Materials are thus obtained. Areducing atmosphere including a reducing gas consisting of at least anyone of generally known reducing gases of hydrogen (H₂), carbon monoxide(CO) and nitrous oxide can be used. In addition, it is preferable thatan additional gas consisting of at least any one of He, Ne, Ar, otherrare gases, nitrogen (N₂) , and carbon dioxide (CO₂) is added further tothe reducing gas because a rate of the reduction and a treatment timefor the reduction can be controlled. A reducing atmosphere explainedabove can be used as either reducing atmosphere explained hereinafter.

It is preferable that the ceramic consisting of lithium tantalate orother mixed oxide with nonstoichiometric composition is used because ithas lack of cations and so it has high reducing property. It ispreferable to use a composition that doesn't have a composition ratioaccording to stoichiometry, for example, lithium tantalate in which acomposition ratio of lithium and tantalum is not 50:50, because it has alot of lack of cations.

In addition, there is a lithium-tantalate crystal as an example ofmaterials used in the present invention. This can be grown by puttingthe ceramic consisting of lithium tantalate before being subjected toreduction treatment into a crucible made of noble metals, heating andmelting the ceramic, and then pulling a crystal using a seed crystalwith being rotated (so-called Czochralski method). Furthermore, alithium-niobate crystal can be grown according to the same method.Although a method to reduce thus-obtained lithium-tantalate crystal isthe same as that for the above-mentioned ceramic, it is preferable thatan atmosphere is introduced into the furnace at a temperature of 250° C.or less during lowering temperature after the heat treatment, and thatthe crystal is removed from the furnace when the temperature becomes 30°C. or less. It is not preferable to introduce an atmosphere at atemperature higher than 250° C. because there-is a possibility that thelithium-tantalate crystal is oxidized. It is also preferable to use acomposition that has nonstoichiometric composition and that doesn't havea composition ratio according to stoichiometry as the crystal, as is thecase with the ceramic.

There is a sliced wafer or a lapped wafer consisting of alithium-tantalate crystal as an example of materials used in the presentinvention. For example; above-mentioned lithium-tantalate crystal, forexample, having a diameter of 100 mm is sliced, for example, by a wiresaw, thereby a wafer subjected to a slicing process having a diameter of100 mm and a thickness of 0.5 mm is obtained. Furthermore, by processingthis wafer using a lap machine, a-wafer subjected to a lapping processhaving a diameter of 100 mm and a thickness of 0.4 mm is obtained. Apolished wafer obtained by polishing one side or both sides of thelapped wafer can also be used. A method to reduce thus-obtained wafer isthe same as that for the above-mentioned crystal. The crystalorientation of the wafer can be selected according to thecharacteristics to be required, for example, a wafer oriented with thesurface normal to the 36° rotated y-direction can be used.

By the reduction treatment, color of a lithium-tantalate crystal or alithium-tantalate wafer as materials changes from white before treatmentto black and the crystal or the wafer gains absorptive power. However,because the temperature of T1, T1′ and T1″ are not less than the Curietemperature of a lithium-tantalate crystal, a lithium-tantalate crystalor a lithium-tantalate wafer obtained by this treatment has amulti-domain structure unsuitable for the SAW device.

In addition, other than those described above, other reduced crystals, aceramic, a hydrogen occluded metal like palladium, preferably a hydrogenstorage alloy in which niobium (Nb), manganese (Mn) and the like aremixed with a basic element such as LaNi₅, FeTi or Mg₂Ni are also used asthe materials in the present invention.

Next, a lithium-tantalate crystal or a lithium-tantalate wafer of whichelectric conductivity is finally increased in the present invention canbe prepared by the same way as the lithium-tantalate crystal or thelithium-tantalate wafer to be the materials. That is by growing acrystal by Czochralski method or slicing process and lapping process forthe grown crystal. In order to single-polarize these, a noble metalelectrode is set on the lithium-tantalate crystal, then a voltage isapplied at a temperature of not lower than the Curie temperature oflithium tantalate, for example at 650° C. Thereby, a single polarizationprocess is conducted. Furthermore, by subjecting this single-polarizedcrystal to slicing process or lapping process, a single-polarized slicedwafer or lapped wafer is obtained. A polished wafer obtained bypolishing one side or both sides of the lapped wafer can also be used.

Next, the lithium-tantalate crystal that is subjected to the singlepolarization process as mentioned above is superposed on a substancereduced at a temperature of T1 or a first material to contact with it.The crystals and the substances or the first materials are piledalternately and placed in a furnace through which, for example, areducing gas is flowed at a rate of approximately 1.5 liters per minute.A temperature of the furnace is increased from room temperature at arate of around 6.7 ° C. per minute. After maintaining the temperature ata temperature of T2 lower than T1 or a temperature of T2′ lower than theCurie temperature, for example from 400° C.-600° C. for 1-50hours, thetemperature of the furnace is decreased at a rate of around 6.7 ° C. perminute. An atmosphere is introduced into the furnace at a temperature of250° C. or less, and the crystal is removed from the furnace when thetemperature becomes 30° C. or less.

As described above, according to the present invention, alithium-tantalate crystal with single-polarized structure and withincreased electric conductivity can be obtained. Moreover, according tothe present invention, in order to surely obtain a lithium-tantalatecrystal with the further increased electric conductivity, eitherfollowing two methods can be conducted in addition to theabove-mentioned methods.

The first method is explained as follows. First, a lithium-tantalatecrystal with single-polarized structure and with increased electricconductivity is placed in a furnace in which an atmosphere isintroduced. A temperature of the furnace is increased from roomtemperature at a rate of around 6.7° C. per minute. After maintainingthe temperature at a temperature of T3 that is lower than Curietemperature, for example, 400° C.-600° C. for 1-50 hours in theatmosphere, the temperature of the furnace is lowered at a rate ofaround 6.7° C. per minute, and the crystal is removed from the furnacewhen the temperature becomes 30° C. or less. By conducting such a heattreatment, the lithium-tantalate crystal becomes white and its electricconductivity once decreases and its resistivity increases. However,activated state to a reduction reaction in the crystal is maintained andthe single polarized structure is also maintained.

Next, the white lithium-tantalate crystal subjected to the heattreatment in the atmosphere as mentioned above is superposed on thesecond material obtained by being subjected to the heat treatment at thetemperature of T1″ as mentioned above, for example, a wafer of theblackly colored lithium-tantalate crystal to contact with it. Thecrystal and the second material are piled alternately and placed in afurnace through which a reducing gas is flowed at a rate ofapproximately 1.5 liters per minute. A temperature of the furnace isincreased from room temperature at a rate of around 6.7 ° C. per minute.After maintaining the temperature at a temperature of T2″ lower than theCurie temperature, preferably at 400° C.-600° C. for 1-50 hours, thetemperature of the furnace is decreased at a rate of around 6.7 ° C. perminute. An atmosphere is introduced into the furnace at a temperature of250° C. or less, and the crystal is removed from the furnace when thetemperature becomes 30° C. or less. By such a heat treatment, asingle-polarized lithium-tantalate crystal in which electricconductivity is further uniformly increased is surely obtained.Particularly, although the black lithium-tantalate crystal obtained byone time of reduction treatment is prone to be colored unevenly, bybeing subjected to two times of reduction, the uniform blacklithium-tantalate crystal without unevenness of color corresponding tovariation of electric conductivity can be surely obtained.

On the other hand, the second method is explained as follows. Activatedstate to a reduction reaction is maintained in the whitelithium-tantalate crystal subjected to the heat treatment in anatmosphere. Accordingly, the following heat treatment can be conducted:without using the second material, the crystal is placed alone in afurnace. A reducing gas is flowed at a rate of approximately 1.5 litersper minute. A temperature of the furnace is increased from roomtemperature at a rate of around 6.7 ° C. per minute. After maintainingthe temperature at a temperature of T2′″ lower than the Curietemperature, preferably at 400° C.-600° C. for 1-50 hours, thetemperature of the furnace is decreased at a rate of around 6.7 ° C. perminute. An atmosphere is introduced into the furnace at a temperature of250° C. or less, and the crystal is removed from the furnace when thetemperature becomes 30° C. or less. By such a heat treatment, thesingle-polarized black lithium-tantalate crystal in which electricconductivity is further uniformly increased and there is no unevennessof color corresponding to variation of electric conductivity can also besurely obtained.

The electric conductivity of the single-polarized lithium-tantalatecrystal with increased electric conductivity obtained by the presentinvention can be measured as follows. That is, the electric conductivityis a reciprocal of a volume resistivity, and the volume resistivity canbe obtained from a resistance measured by using 4329A High ResistanceMeter and 16008A Resistivity Cell manufactured by Hewlett Packard,according to the following formula.ρ=(πd²/4t)·Rρ: Volume resistivity (Ω·cm)Ω: the ratio of the circumference to its diameterd : Diameter of center electrode (cm)t : Thickness of a lithium-tantalate crystal or a lithium-tantalatewafer (cm)R : Resistance (Ω)

In measurement, the voltage of, for example 500 V may be applied to awafer, and the resistance may be measured one minute after theapplication of voltage in order to obtain a stable measurement value.

In addition, effect of the present invention is also confirmed bymeasuring surface potential of a wafer. Surface potential is a quantityof electric charge accumulated on the surface by temperature differencedue to pyroelectric property. This is known as measurement of surfacepotential that is the same quantitative measurement as measurement ofstatic electricity. While a temperature of a lithium-tantalate waferwith increased electric conductivity is increased from 30° C.-70° C. in1 minute on a hot plate, change of surface potential during that time ismeasured, for example, by using SFM775 manufactured by Ion Systems,surface potential is thus obtained.

Hereinafter, the present invention will be explained in detail inreference to examples and comparative examples, however, the presentinvention is not limited thereto.

A lithium-tantalate (LT) wafer to be used in examples and comparativeexamples was produced as follows. A lithium-tantalate crystal orientedwith the surface normal to the 36 rotated y-direction and having adiameter of 100 mm and a length of 50 mm was obtained by usingCzochralski method and commonly used secondary processing method(hereinafter referred to as an LT crystal). A platinum electrode was seton-the LT crystal, a voltage was applied at 650° C. and a singlepolarization process was conducted. Next, this LT crystal was subjectedto a slicing process and a lapping process, and double-sided lappedwafers having a thickness of 0.4 mm were obtained (hereinafter thesewafers are referred to as LT lapped wafers). One side of these LT lappedwafers was polished and wafers having a thickness of 0.35 mm wereobtained (hereinafter these wafers are referred to as LT polishedwafers). These wafers were colorless and translucent.

EXAMPLES 1-18

The LT lapped wafers thus obtained were placed in a sealed furnacethrough which a reducing gas shown in Table 1 was flowed at a rate ofapproximately 1.5 liters per minute. This furnace comprised a three-zonetube furnace with a horizontal, 200 mm-diameter alumina process tube.The wafers were supported by alumina carriers placed in the center ofthe process tube. The alumina process tube extended out of the furnaceso that its ends were exposed and remained cool. O-ring seals on thealumina process tube provided a sealed furnace cavity. The wafer wasloaded into the process tube, which was then sealed with end caps. Thegas flow was initiated and the furnace heating begun. The temperature ofthe furnace was increased from room temperature to the temperature ofT1shown in Table 1 at a rate of about 6.7° C. per minute. Then, thetemperature of T1 was maintained for 1 hour, the furnace was cooled downat a rate of about 6.7° C. per minute. An atmosphere was introduced intothe furnace when the temperature reached 250° C. or less, and the waferis removed from the furnace when the temperature reached 30° C. or less(hereinafter this wafer is referred to as a T1-processed LT wafer).Next, an LT lapped wafer subjected to a single polarization process andthe T1-processed LT wafer were piled alternately to be contacted witheach other, and the wafers were placed in a sealed furnace through whicha hydrogen gas was flowed at rate of approximately 1.5 liters perminute. This furnace is the same as that used in the reduction treatmentat the temperature of T1. The temperature of the furnace was increasedfrom room temperature at a rate of about 6.7° C. per minute. After thefurnace was maintained at the temperature of T2 shown in Table 1 for 1hour, the furnace was cooled down at a rate of about 6.7° C. per minute.An atmosphere was introduced into the furnace when the temperaturereached 250° C. or less, and the wafer was removed from the furnace whenthe temperature reached 30° C. or less (hereinafter this wafer isreferred to as a T2-processed LT wafer).

The electric conductivity of the T2-processed LT wafer was obtained asfollows. The electric conductivity is a reciprocal of a volumeresistivity, so at first the volume resistivity was obtained from aresistance measured by 4329A High Resistance Meter and 16008AResistivity Cell manufactured by Hewlett Packard according to thefollowing formula.ρ=(πd²/4t)·Rρ: Volume resistivity (Ω·cm)Ω: the ratio of the circumference to its diameterd : Diameter of center electrode (cm)t : Thickness of a lithium-tantalate crystal or a lithium-tantalatewafer (cm)R : Resistance (Ω)

In this measurement, the voltage of 500 V was applied to the wafer, andthe resistance was measured one minute after the application of voltage,in order to obtain a stable measurement value.

The surface potential was measured as follows. The T2-processed LT waferwas heated up on a hot plate from 30° C. to 70° C. in 1 minute, and themeasurement was obtained by using SFM775 manufactured by Ion Systems asthe change of surface potential during the heating-up. The electricconductivity and the surface potential thus obtained are shown inTable 1. In all the following Tables, description such as “9.3E−14” inelectric conductivity, means “9.3×10⁻¹⁴.” As a result, in every case ofexamples 1-18, the electric conductivity was high and the surfacepotential was low. In order to further increase the electricconductivity and to further lower the surface potential, it waspreferable that the temperature of T1 was 700° C. or higher. And inevery example, because T2 was Curie temperature or lower,single-polarized structure of the T2-processed LT wafer was maintained.

EXAMPLES 19-28

The LT lapped wafers were placed in a sealed furnace through which ahydrogen gas was flowed at a rate of approximately 1.5 liters perminute. The wafer was maintained at the temperature of T1shown in Table2 for 1 hour, and a T1-processed LT wafer was obtained. Then, thesingle-polarized LT lapped wafer and the T1-processed LT wafer werepiled alternately to be contacted with each other, and placed in asealed furnace through which a reducing gas shown in Table 2 was flowedat a rate of approximately 1.5 liters per minute. After the furnace wasmaintained at the temperature of T2 shown in Table 2 for 1 hour, thefurnace was cooled down at a rate of about 6.7° C. per minute. Anatmosphere was introduced into the furnace when the temperature reached250° C. or less, and the wafer was removed from the furnace when thetemperature reached 30° C. or less. Thereby, a T2-processed LT wafer wasobtained. The electric conductivity and surface potential measuredaccording to the same method as set forth was shown in Table 2. As aresult, in every case of examples 19-28, the electric conductivity washigh and the surface potential was low. In order to further increase theelectric conductivity and to further lower the surface potential, it waspreferable that the temperature of T2 was 400° C. or higher. And inevery example, because T2 was Curie temperature or lower,single-polarized structure of the T2-processed LT wafer was maintained.

Comparative Examples 1-4

The electric conductivity and surface potential of the LT lapped wafersthat were never subjected to reduction treatment is shown in Table 3.The electric conductivity was low and the surface potential was high.

The LT lapped wafers were placed in a sealed furnace through which ahydrogen gas was flowed at a rate of approximately 1.5 liters perminute. After the wafer was maintained at the temperature of 600° C.that is not more than Curie temperature for the time shown in Table 3,the furnace was cooled down at a rate of about 6.7° C. per minute. Whenthe temperature reached 30° C. or less, an atmosphere was introducedinto the furnace and the wafer was removed from the furnace. Thereby, areduced LT wafer was obtained. The electric conductivity and surfacepotential measured according to the same method as set forth was shownin Table 3. Even when any heat treatment time was used, both theelectric conductivity and the surface potential were nearly equal valuesto those of the LT wafer that was not reduced. In addition, color changeof the wafer was not observed by visual inspection.

EXAMPLES 29-31

The LT lapped wafers were placed in a sealed furnace through which ahydrogen gas was flowed at a rate of approximately 1.5 liters perminute. The wafer was maintained at 1000° C. for 10 hours, thereby theT1-processed LT wafer was obtained. Then, the LT wafer and theT1-processed LT wafer were piled alternately to be contacted with eachother, and the wafer was placed in a sealed furnace through which ahydrogen gas was flowed at a rate of approximately 1.5 liters perminute. After the furnace was maintained at 550° C. for 6 hours, thefurnace was cooled down at a rate of about 6.7° C. per minute. Anatmosphere was introduced into the furnace when the temperature reached250° C. or less, and the wafer was removed from the furnace when thetemperature reached 30° C. or less. Thereby, three slices ofT2-processed LT wafers were obtained. Next, two slices of wafers amongthe T2-processed LT wafers were placed in a furnace in which anatmosphere was introduced, the furnace was maintained at 550° C. for 6hours and T3-processed LT wafers were obtained. Then, one slice of theT3-processed LT wafers and another T1-processed LT wafer were piledalternately to be contacted with each other, and placed in a sealedfurnace through which a hydrogen gas was flowed at a rate ofapproximately 1.5 liters per minute. After the furnace was maintained at550° C. for 6 hours, the furnace was cooled down at a rate of about 6.7°C. per minute. An atmosphere was introduced into the furnace when thetemperature reached 250° C. or less, and the wafer was removed from thefurnace when the temperature reached 30° C. or less. Thereby, aT2″-processed LT wafer was obtained. Then, another slice of theT3-processed LT wafers was placed in a sealed furnace through which ahydrogen gas was flowed at a rate of approximately 1.5 liters perminute. After the furnace was maintained at 550° C. for 6 hours, thefurnace was cooled down at a rate of about 6.7° C. per minute. Anatmosphere was introduced into the furnace when the temperature reached250° C. or less, and the wafer was removed from the furnace when thetemperature reached 30° C. or less. Thereby, a T2′″-processed LT waferwas obtained. The electric conductivity and surface potential of theT2-processed LT wafer, the T2″-processed LT wafer and the T2′″-processedLT wafer measured according to the same method as set forth were shownin Table 4. As a result, in examples 30and 31, the electric conductivitywas higher and the surface potential was lower than those in example 29.

The present invention is not limited to the above-described embodiments.The above-described embodiments is mere examples, and those having thesubstantially same structure as that described in the appended claimsand providing the similar action and effects are included in the scopeof the present invention. TABLE 1 Electric T1 reducing TemperatureTemperature Conductivity Surface atmosphere T1 (° C.) T2 (° C.)(Ω⁻¹cm⁻¹) Potential (kV) Example 1 100% H₂ 1100 600 9.3E−12 <0.1 Example2 100% H₂ 1000 600 7.7E−12 <0.1 Example 3 100% H₂ 900 600 6.2E−12 <0.1Example 4 100% H₂ 800 600 4.8E−12 <0.1 Example 5 100% H₂ 700 600 3.2E−12<0.1 Example 6 100% H₂ 650 600 1.4E−13 1.3 Example 7 100% CO 700 6003.6E−12 <0.1 Example 8 90% H₂-10% N₂ 700 600 1.8E−12 <0.1 Example 9 10%H₂-90% N₂ 700 600 1.2E−12 <0.1 Example 10 10% H₂-90% He 700 600 2.9E−12<0.1 Example 11 10% H₂-90% Ne 700 600 2.2E−12 <0.1 Example 12 10% H₂-90%Ar 700 600 3.1E−12 <0.1 Example 13 100% H₂ 1100 500 6.8E−12 <0.1 Example14 100% H₂ 1100 400 2.6E−12 <0.1 Example 15 100% H₂ 1100 350 1.5E−13 1.2Example 16 100% CO 1100 500 6.7E−12 <0.1 Example 17 100% CO 400 3.5E−12<0.1 Example 18 100% CO 1100 350 2.0E−13 0.9

TABLE 2 Electric Surface T2 reducing Temperature TemperatureConductivity Potential atmosphere T1 (° C.) T2 (° C.) (Ω⁻¹cm⁻¹) (kV)Example 19 100% CO 1100 600 8.8E−12 <0.1 Example 20 100% CO 1100 5006.0E−12 <0.1 Example 21 100% CO 1100 400 2.9E−12 <0.1 Example 22 100% CO1100 350 1.6E−13 1.4 Example 23 100% CO 700 600 2.8E−12 <0.1 Example 2490% H₂-10% N₂ 700 600 1.9E−12 <0.1 Example 25 10% H₂-90% N₂ 700 6001.1E−12 <0.1 Example 26 10% H₂-90% He 700 600 3.2E−12 <0.1 Example 2710% H₂-90% Ne 700 600 2.3E−12 <0.1 Example 28 10% H₂-90% Ar 700 6002.3E−12 <0.1

TABLE 3 Electric Temperature Time Conductivity Surface T1 (° C.) (hr)(Ω⁻¹cm⁻¹) Potential (kV) Comparative without reduction — 3.0E−15 6.7Example 1 treatment Comparative 600 1 6.8E−15 6.5 Example 2 Comparative600 10 7.7E−15 6.5 Example 3 Comparative 600 20 8.5E−15 6.4 Example 4

TABLE 4 Electric Surface Conductivity Potential Wafer (Ω⁻¹cm⁻¹) (kV)Example 29 T2′-processed LT wafer 8.5E−12 <0.1 Example 30 T2″-processedLT wafer 1.5E−11 <0.1 Example 31 T2′″-processed LT wafer 1.4E−11 <0.1

1-26. (canceled)
 27. A method of producing a lithium-tantalate crystalwith increased electric conductivity, wherein a substance reduced at atemperature of T2 is contacted with a lithium-tantalate crystal at atemperature of T2 that is lower than the temperature of T1 in a reducingatmosphere.
 28. The method of producing a lithium-tantalate crystalaccording to claim 27, wherein the temperature of T1 is 700° C. orhigher.
 29. The method of producing a lithium-tantalate crystalaccording to claim 27, wherein the reduction at the temperature of T1 isperformed in a reducing gas containing any one of hydrogen, carbonmonoxide and nitrous oxide, or a mixed gas consisting of two or more ofthem.
 30. The method of producing a lithium-tantalate crystal accordingto claim 29, wherein the reduction is performed in an atmosphere inwhich any one of He, Ne, Ar, other rare gases, nitrogen, and carbondioxide, or a mixed gas consisting of two or more of them is addedfurther to the reducing gas.
 31. The method of producing alithium-tantalate crystal according to claim 27, wherein any one of acrystal, a ceramic and a metal is used as the substance reduced at thetemperature of T1.
 32. The method of producing a lithium-tantalatecrystal according to claim 28, wherein any one of a crystal, a ceramicand a metal is used as the substance reduced at the temperature of T1.33. The method of producing a lithium-tantalate crystal according toclaim 29, wherein any one of a crystal, a ceramic and a metal is used asthe substance reduced at the temperature of T1.
 34. The method ofproducing a lithium-tantalate crystal according to claim 30, wherein anyone of a crystal, a ceramic and a metal is used as the substance reducedat the temperature of T1.
 35. The method of producing alithium-tantalate crystal according to claim 31, wherein the crystal orthe ceramic which consists of mixed oxide with nonstoichiometriccomposition is used.
 36. The method of producing a lithium-tantalatecrystal according to claim 31, wherein a lithium tantalate or a lithiumniobate is used as the crystal or the ceramic.
 37. The method ofproducing a lithium-tantalate crystal according to claim 32, wherein alithium tantalate or a lithium niobate is used as the crystal or theceramic.
 38. The method of producing a lithium-tantalate crystalaccording to claim 33, wherein a lithium tantalate or a lithium niobateis used as the crystal or the ceramic.
 39. The method of producing alithium-tantalate crystal according to claim 34, wherein a lithiumtantalate or a lithium niobate is used as the crystal or the ceramic.40. The method of producing a lithium-tantalate crystal according toclaim 35, wherein a lithium tantalate or a lithium niobate is used asthe crystal or the ceramic.
 41. The method of producingalithium-tantalate crystal according to claim 31, wherein a hydrogenstorage alloy is used as the metal.
 42. The method of producing alithium-tantalate crystal according to claim 27, wherein asingle-polarized crystal is used as the lithium-tantalate crystalcontacted at the temperature of T2 with the substance reduced at thetemperature of T1.
 43. The method of producing a lithium-tantalatecrystal according to claim 28, wherein a single-polarized crystal isused as the lithium-tantalate crystal contacted at the temperature of T2with the substance reduced at the temperature of T1.
 44. The method ofproducing a lithium-tantalate crystal according to claim 29, wherein asingle-polarized crystal is used as the lithium-tantalate crystalcontacted at the temperature of T2 with the substance reduced at thetemperature of T1.
 45. The method of producing a lithium-tantalatecrystal according to claim 30, wherein a single-polarized crystal isused as the lithium-tantalate crystal contacted at the temperature of T2with the substance reduced at the temperature of T1.
 46. The method ofproducing a lithium-tantalate crystal according to claim 31, wherein asingle-polarized crystal is used as the lithium-tantalate crystalcontacted at the temperature of T2 with the substance reduced at thetemperature of T1.
 47. The method of producing a lithium-tantalatecrystal according to claim 32, wherein a single-polarized crystal is-used as the lithium-tantalate crystal contacted at the temperature ofT2 with the substance reduced at the temperature of T1.
 48. The methodof producing a lithium-tantalate crystal according to claim 33 wherein asingle-polarized crystal is used as the lithium-tantalate crystalcontacted at the temperature of T2 with the substance reduced at thetemperature of T1.
 49. The method of producing a lithium-tantalatecrystal according to claim 34, wherein a single-polarized crystal isused as the lithium-tantalate crystal contacted at the temperature of T2with the substance reduced at the temperature of T1.
 50. The method ofproducing a lithium-tantalate crystal according to claim 35, wherein asingle-polarized crystal is used as the lithium-tantalate crystalcontacted at the temperature of T2 with the substance reduced at thetemperature of T1.
 51. The method of producing a lithium-tantalatecrystal according to claim 36, wherein a single-polarized crystal isused as the lithium-tantalate crystal contacted at the temperature of T2with the substance reduced at the temperature of T1.
 52. The method ofproducing a lithium-tantalate crystal according to claim 37, wherein asingle-polarized crystal is used as the lithium-tantalate crystalcontacted at the temperature of T2 with the substance reduced at thetemperature of T1.
 53. The method of producing a lithium-tantalatecrystal according to claim 38, wherein a single-polarized crystal isused as the lithium-tantalate crystal contacted at the temperature of T2with the substance reduced at the temperature of T1.
 54. The method ofproducing a lithium-tantalate crystal according to claim 39, wherein asingle-polarized crystal is used as the lithium-tantalate crystalcontacted at the temperature of T2 with the substance reduced at thetemperature of T1.
 55. The method of producing a lithium-tantalatecrystal according to claim 40, wherein a single-polarized crystal isused as the lithium-tantalate crystal contacted at the temperature of T2with the substance reduced at the temperature of T1.
 56. The method ofproducing a lithium-tantalate crystal according to claim 41, wherein asingle-polarized crystal is used as the lithium-tantalate crystalcontacted at the temperature of T2 with the substance reduced at thetemperature of T1.
 57. The method of producing a lithium-tantalatecrystal according to claim 42, wherein a crystal that is in a stepbefore slicing is used as the single-polarized crystal.
 58. The methodof producing a lithium-tantalate crystal according to claim 42, whereina wafer subjected to a slicing process or a wafer subjected to a lappingprocess is used as the single-polarized crystal.
 59. The method ofproducing a lithium-tantalate crystal according to claim 27, wherein thetemperature of T2 is 400° C.-600° C.
 60. The method of producing alithium-tantalate crystal according to claim 59, wherein after theprocess at the temperature of T2, an atmosphere is introduced at atemperature of 250° C. or less.
 61. The method of producing alithium-tantalate crystal according to claim 27, wherein the reductionat the temperature of T2 is performed in a reducing gas consisting ofany one of hydrogen, carbon monoxide and nitrous oxide, or a mixed gasconsisting of them.
 62. The method of producing a lithium-tantalatecrystal according to claim 61, wherein the reduction is performed in anatmosphere in which an inert gas consisting of He, Ne, Ar, other raregases, nitrogen, and carbon dioxide, or a mixed gas consisting of themis added further to the reducing gas.
 63. A method of producing alithium-tantalate crystal, wherein at least a first material containinglithium tantalate, lithium niobaie or hydrogen storage alloy storinghydrogen that is subjected to a heat treatment at a temperature of T1′that is Curie temperature or higher in a reducing atmosphere issuperposed on a single-polarized lithium-tantalate crystal, and then thecrystal is subjected to a heat treatment at a temperature of T2′ that islower than Curie temperature in a reducing atmosphere, thereby anelectric conductivity of the single-polarized lithium-tantalate crystalis increased.
 64. A method of producing a lithium-tantalate crystal,wherein the lithium-tantalate crystal obtained by the method accordingto claim 63 is subjected to a heat treatment at a temperature of T3 thatis lower than Curie temperature in an atmosphere, the lithium-tantalatecrystal is superposed on a second- material containing lithiumtantalate, lithium niobate or hydrogen storage alloy storing hydrogenthat is subjected to a heat treatment at a temperature of T1″ that isCurie temperature or higher in a reducing atmosphere, and the crystal issubjected to a heat treatment at a temperature of T2″ that is lower thanCurie temperature in a reducing atmosphere, thereby an electricconductivity of the lithium-tantalate crystal is increased.
 65. A methodof producing a lithium-tantalate crystal, wherein the lithium-tantalatecrystal obtained by the method according to claim 63 is subjected to aheat treatment at a temperature of T3 that is lower than Curietemperature in an atmosphere, the lithium-tantalate crystal is subjectedto a heat treatment at a temperature of T2′″ that is lower than Curietemperature in a reducing atmosphere, thereby an electric conductivityof the lithium-tantalate crystal is increased.
 66. The method ofproducing a lithium-tantalate crystal according to claim 63, wherein thetemperatures of T2′ that is lower than Curie temperature are 400° C. orhigher.
 67. The method of producing a lithium-tantalate crystalaccording to claim 63, wherein a wafer subjected to a slicing process ora wafer subjected to a lapping process is used as the single-polarizedlithium-tantalate crystal.
 68. The method of producing alithium-tantalate crystal according to claim 63, wherein a ceramic or acrystal consisting of lithium tantalate or lithium niobate is used asthe first material.
 69. The method of producing a lithium-tantalatecrystal according to claim 64, wherein a ceramic or a crystal consistingof lithium tantalate or lithium niobate is used as the first material orthe second material.
 70. The method of producing a lithium-tantalatecrystal according to claim 65, wherein a ceramic or a crystal consistingof lithium tantalate or lithium niobate is used as the first material.71. The method of producing a lithium-tantalate crystal according toclaim 66, wherein a ceramic or a crystal consisting of lithium tantalateor lithium niobate is used as the first material.
 72. The method ofproducing a lithium-tantalate crystal according to claim 67, wherein aceramic or a crystal consisting of lithium tantalate or lithium niobateis used as the first material.
 73. The method of producing alithium-tantalate crystal according to claim 68, wherein a compositionthat doesn't have a composition ratio according to stoichiometry is usedas the ceramic or the crystal consisting of lithium tantalate or lithiumniobate.
 74. The method of producing a lithium-tantalate crystalaccording to claim 68, wherein a wafer subjected to a slicing processthat is obtained from a crystal consisting of lithium tantalate orlithium niobate or a wafer subjected to a lapping process that isobtained from a crystal consisting of lithium tantalate or lithiumniobate is used as the first material.
 75. The method of producing alithium-tantalate crystal according to claim 63, wherein after the heattreatment at the temperature of T2′, the lithium-tantalate crystal isexposed to an atmosphere at a temperature of 250° C. or less.
 76. Themethod of producing a lithium-tantalate crystal according to claim 63,wherein the reducing atmosphere for the heat treatment at thetemperature of T1′ and T2′ contains a reducing gas consisting of atleast any one of hydrogen, carbon monoxide and nitrous oxide.
 77. Themethod of producing a lithium-tantalate crystal according to claim 76,wherein an additional gas consisting of at least any one of rare gases,nitrogen, and carbon dioxide is added further to the reducing gas.